1. Field of the Invention:
This invention relates to a catalyst for the production of pyromellitic anhydride and a method for the production of pyromellitic anhydride. More particularly, it relates to a catalyst to be used in the production of pyromellitic anhydride by the catalytic vapor-phase oxidation of a tetraalkylbenzene and a method for the production of pyromellitic anhydride from a tetraalkylbenzene by the use of the catalyst. The pyromellitic anhydride is extensively used as heat-resistant resins, plasticizers, epoxy resin-curing agents, etc. As an industrial raw material, it has been acquiring an increasingly great importance in recent years.
2. Description of the Prior Art:
Besides the method of catalytic vapor-phase oxidation of 1,2,4,5-tetramethylbenzene (occasionally referred to briefly as "durene"), a method which resorts to the liquid-phase oxidation of durene, a method which relies on the liquid-phase oxidation of 2,4,5-trimethylbenzaldehyde, and methods which reside in the synthesis of starting materials other than durene have been proposed for the production of pyromellitic anhydride. Among other methods cited above, the method of vapor-phase oxidation of durene has been particularly attracting attention as a process allowing inexpensive quantity production of pyromellitic anhydride because the raw material, durene, which has been heretofore expensive now promises to be inexpensively supplied in large quantities owing to the use of a zeolite type catalyst.
Numerous inventions covering catalysts for the catalytic vapor-phase oxidation of durene have been disclosed in patent literature. For example, a V.sub.2 O.sub.5 --P.sub.2 O.sub.5 --TiO.sub.2, MoO.sub.3, WO3 type catalyst (JP-B-45-4,978), a V.sub.2 O.sub.5 --TiO.sub.2 anatase type)-MoO.sub.3, P.sub.2 O.sub.5 type catalyst (JP-B-45-15,018), a V.sub.2 O.sub.5 --TiO.sub.2 --Na.sub.2 O, P.sub.2 O.sub.5 type catalyst (JP-B-45-15,252), a V.sub.2 O.sub.5 --MoO.sub.3 --P.sub.2 O.sub.5 type catalyst (JP-B-47-38,431), a V.sub.2 O.sub.5 --MoO.sub.3 --TiO.sub.2 type catalyst (JP-B-49-30,821), a V.sub.2 O.sub.5 --TiO.sub.2 --P.sub.2 O.sub.5 --Nb.sub.2 O.sub.5 --K.sub.2 O, CsO type catalyst (JP-A-49-31,972), a V.sub.2 O.sub.5 --B.sub.2 O.sub.3 --SnO.sub.2, P.sub.2 O.sub.5, TiO.sub.2, Na.sub.2 O type catalyst (JP-B-49-31,973), a V.sub.2 O.sub.5 --B.sub.2 O.sub.5 type catalyst (JP-B-48-35,251), and a V.sub.2 O.sub.5 --Na.sub.2 O--MoO.sub.3 --Cr, Mn, Nb type catalyst (U.S. Pat. No. 4,925,957 and U.S. Pat. No. 5,001,100) have been disclosed.
The catalysts of these conventional compositions, however, hardly deserve to be called satisfactory from the commercial point of view because the concentration of a tetraalkylbenzene in the raw material gas composition is as low as less than 20 g/Nm.sup.3 and the pyromellitic acid aimed at is obtained only in an unduly low yield when this concentration is increased beyond 20 g/Nm.sup.3. None of the known catalysts contains silver as a component element. The effect of silver in the catalysis under consideration has never been known to the art.
A catalyst made of a vanadate which contains such a second metallic component as niobium, silver, molybdenum, chromium, or manganese has been known to be useful as a catalyst for the production of pyromellitic anhydride by the vapor-phase oxidation of durene with a molecular oxygen-containing gas (BP 1,147,554). This patent, however, only shows cases of using niobium vanadate and the per-pass yield of the operation involved is as low as about 46% by weight.
JP-B-43-26,497 discloses a catalyst formed of vanadium and niobium and intended for the production of pyromellitic anhydride by the vapor-phase oxidation of durene. The specification of this invention remarks in effect that this vanadium-niobium type catalyst may incorporate additionally therein such promoters as oxides, sulfates, or phosphates of alkali metals and alkaline earth metals, boron, silver, manganese or phosphorus, and antimony or arsenic. None of the working examples cited in this specification is found to embody the addition of such a promoter. Further, none of the operations demonstrating the use of the vanadium-niobium type catalyst is found to have given any sufficient yield.
An object of this invention, therefore, is to provide a novel catalyst for the production of pyromellitic anhydride by the catalytic vapor-phase oxidation of a tetraalkylbenzene.
Another object of this invention is to provide a novel method for the production of pyromellitic anhydride by the catalytic vapor-phase oxidation of a tetraalkylbenzene.
Yet another object of this invention is to provide a catalyst for producing pyromellitic acid efficiently with high yield in terms of commercial operation and a method for the production of pyromellitic anhydride.
Still another object of this invention is to provide a catalyst for producing pyromellitic anhydride at a high raw material gas concentration and with high yield and a method for the production of pyromellitic anhydride.
Yet still another object of this invention is to provide a product without color, to enhance high catching effect, and to attain synthetically high yield.